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Sports Direct normally sell them, the website only has Campingaz but I've always been able to get normal screw in canisters in the shops.
You could also try giving Snow Peak on Regents St a call, I've no idea if they have stock but it feels like somewhere that doesn't get loads of foot traffic and might have some.
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Do they make adapters for the Campingaz carts? That would be another option.
EDIT: looks like maybe yes: https://mercatorgear.com/product/kovea-adapter-pack-bundle/
Looks like there's a shortage of camping gas cartridges too. Anyone know where there's any in stock (other than those stupid Campingaz Clic ones) ?